Dimeric anthocyanins from strawberry (Fragaria ananassa) consisting of pelargonidin 3-glucoside covalently linked to four flavan-3-ols

摘要

Flavanol–anthocyanin complexes were isolated by successive use of Amberlite XAD-7 chromatography, Sephadex LH-20 gel filtrationand preparative HPLC from acidified, methanolic extract of strawberries (Fragaria ananassa Dutch.). These purple minorpigments were characterized by UV–Vis spectroscopy, 1D and 2D NMR techniques, and electrospray mass spectrometry to be catechin(4a ! 8)pelargonidin 3-O-b-glucopyranoside (1), epicatechin(4a ! 8)pelargonidin 3-O-b-glucopyranoside (2), afzelechin(4a !8)pelargonidin 3-O-b-glucopyranoside (3) and epiafzelechin(4a ! 8)pelargonidin 3-O-b-glucopyranoside (4). The stereochemistry atthe 3- and 4-positions of the flavan-3-ol units was based on assumption ofR-configuration at C-2. Each of the four pigments occurred intheNMR solvent as a pair of rotamers. Proved by cross-peaks in the 1H–1HNOESYNMRspectra of 1, 2 and 4, the two conformationswithin each rotameric pair were in equilibrium with each other. Even though 1 and 2 are based on a different aglycone, their structuresmay be similar to tentatively identified pigments, which have been assumed to contribute to the colour of red wines.